Mattress to replace Hilary devey diamond luxe mattress

We bought a Hilary Devey diamond luxe super king mattress about 8 months ago. Unfortunately it's just too hard. I tend to wake up frequently and have to move because I'm sore and uncomfortable.
The mattress is on a padded-top divan base. I'm a side-sleeper, weigh 10.5 stone and am 5'6" tall. I'm female and have quite tender joints and sometimes low-back pain. I'm also a hot-body in bed. And I need my sleep!
Having spent a large amount on the mattress it's really disappointing, but we've got to the stage of accepting we need a good night's sleep. Would a topper suffice? Which of your mattresses and/or toppers would you recommend?
Any advice gratefully accepted.